The invention relates to a roll for guiding a material web in a machine, in particular a paper or board web within a papermaking or coating machine, comprising a roll or drum comprising a jacket and two end covers provided with bearing journals, and the jacket has a number of passage openings through it.
Known web guide devices comprise a drilled or perforated roll or a drum which guides a web. During start-up of the machine or in the event of a break in the material web, a narrow material web strip, also called a threading strip is guided over the roll. During normal operation of the machine, the entire material web is guided over the roll. The roll is connected to a suction device which applies a negative pressure to the interior of the roll. The connection is via a drilled hole in the bearing journal of the roll end cover. The negative pressure in the roll is transferred through the outer surface of the roll jacket through the openings which penetrate the roll jacket. This avoids fluttering or disturbed running of the material web.
Known rolls have disadvantages including eddy losses in the air flow in the interior of the roll and friction losses in the bearing journal drilled hole. As machine speeds rise, the disadvantages become so large that it is no longer possible to apply adequate vacuum in the interior of the roll. To remedy this, the cross section of the drilled hole arranged in the bearing journal must be enlarged. However, enlargement is not arbitrarily possible. As a result, the known roll cannot be used at high machine speeds.
Furthermore, in some known rolls having air passage openings in the jacket, suction is applied by a suction box at the outer surface of the jacket. This introduces negative pressure into the interior of the roll. The negative pressure is propagated via the air passage openings in the jacket over the region of the roll that is covered by the material web. Because of the high space requirement of devices with such suction boxes and because space is not available in every machine, possible uses of the devices are restricted.